A tank-venting arrangement generally includes a fuel tank and a tank-venting valve which is connected to the air-intake pipe of an internal combustion engine so that fuel vapors can be drawn off with the aid of the underpressure in the intake pipe. Conventionally, the volume in the tank disposed above the fuel is not drawn off directly; instead, an adsorption filter, usually an active charcoal filter, is connected between the tank and the tank-venting valve. This active charcoal filter adsorbs fuel in those time durations in which no suction takes place from the intake pipe, for example, when the internal combustion engine is at standstill or, when, because of the actual operating condition, the tank-venting valve is held closed.
The danger is present that the tank-venting arrangement develops a leak or that the tank-venting valve does not operate properly. For this reason, such arrangements are to be checked repeatedly with respect to operability during the operation of a motor vehicle.
The most important method for checking the operability of a tank-venting arrangement of a motor vehicle is based upon a suggestion of the California Environmental Authority CARB. Pursuant to this method, a check is made while opening the tank-venting valve as to whether a lambda controller must undertake a correction of its output value. This is always the case when air with fuel vapor is drawn by suction from the tank-venting arrangement. However, the adsorption filter can be completely regenerated and the fuel in the tank can be completely degassed. Then, when the tank-venting valve is opened, no fuel in addition to that fuel is supplied which is supplied in accordance with the output value of the lambda control to the injection valves of the internal combustion engine. In such a case, in which no fuel is supplied from the tank-venting arrangement, that is where the lambda controller does not undertake a correction, it is unclear as to whether the tank-venting arrangement leaks or whether no fuel is supplied for the reasons mentioned above. In order to decide this question, an evaluation of the signal from the lambda control is made according to the known method only when a fuel temperature sensor indicates that a pregiven fuel minimum temperature is exceeded and a fuel tank sensor indicates that the motor vehicle was tanked. The premise is taken that fuel vapor must be present in the arrangement in any event which is drawn in by suction when the tank-venting valve is opened and then leads to a correction of the lambda controller. However, erroneous decisions always occur with this method when degassed fuel is disposed in the tank, when such fuel is added with a subsequent tanking and when the adsorption filter is substantially regenerated.
Accordingly, the problem continued to be present to provide a method for checking the operability of a motor vehicle tank-venting arrangement which supplies the least amount of unjustified fault announcements possible. Furthermore, the problem was present to provide a tank-venting arrangement the operability of which could be especially reliably checked.